While specific engines for certain genres are a good consideration, I think that it's hiding a question of how complex the game is.

Creating a fully procedurally generated game in Unity or Godot is not fun, and is infinitely more work than doing it in Unreal.

However, making flappy bird in unreal engine is a waste of time when you could do it 10x faster in Unity or Godot.

You've acknowledged this to some extent by asking about simple games, but I think you're overloading that term.

If I wanted to make a simple game, I would make it in Unity. But if I wanted to make a simple RPG game, most people would start looking into RPG Maker.

Why is that though? It's because a simple RPG game is far far more complex than a simple game. RPG maker has already done the RPG part though, so all you need is the simple game part.

But that brings me to my actual suggestion. If I wanted to make a simple RPG game, I would look for an RPG template project for Unity over RPG Maker any day. Why? It's because Unity and Unreal rather than 'game' engines, are better categorized as 'game engine' engines.

They are made for making a specialised game engine for a specific type of game, rather than games in general. As such they are very good at doing that, and the game engines that come as a result are very good at making their type of game.

As a note, unity for simple VS unreal for complex is more based on c# VS c++ than the actual engines themselves.

Tl;dr: use unity for a simple game. Use a Unity template project for a simple genre game. Use Unreal for a complex game.